1. Field of the Invention
The present invention relates to thermoplastic compositions comprising vinylaromatic copolymers, prepared by the emulsion polymerization method, which comprise salt inclusions resulting from the preparative process, the mouldings produced from the compositions being distinguished by an improved surface quality after heat-moisture storage and preferably having an ageing-resistant class A surface free of visual defects.
The present invention further relates to a process for the preparation of the compositions according to the invention, to the use of the compositions according to the invention for the production of mouldings, and to mouldings which have been produced from the compositions according to the invention.
2. Description of Related Art
Compositions comprising vinylaromatic copolymers, prepared by the emulsion polymerization method, which comprise salt inclusions resulting from the preparative process are known in the literature. There are various origins of such salt inclusions resulting from the preparative process, examples being emulsifier solutions, polymerization initiator solutions, buffer solutions and precipitant solutions used as auxiliary substances in the emulsion polymerization method, which remain in the material or are not completely removed from the material during the work-up of the polymer, depending on the process.
In particular, the addition of acids and/or salts to precipitate vinyl polymer latices prepared by emulsion polymerization, which is normally carried out in traditional processes, e.g. in EP 459 161 B1, DE 2 021 398 and DE 28 15 098, makes a substantial contribution to the salt load in the final polymer, since removal of these salts from the product by means of downstream process steps (washing) is generally unsatisfactory or requires a great deal of energy and water/waste water. Examples of coagulants used are preferably aqueous solutions of water-soluble salts such as alkali metal, alkaline earth metal or aluminium chlorides, sulfates, nitrates, phosphates, acetates, formates, aluminates or carbonates, particularly preferably aluminium chloride, calcium chloride and magnesium sulfate solutions, optionally in combination with inorganic or organic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, boric acid, formic acid, acetic acid, propionic acid and citric acid.
According to the literature, such salt inclusions in compositions comprising vinylaromatic copolymers can lead to unwanted effects.
WO 2009/071537, for example, discloses that magnesium and/or calcium compounds in impact strength modified vinylaromatic copolymers selected from the group comprising acrylonitrile-butadiene-styrene (ABS) copolymers, acrylonitrile-styrene-acrylate (ASA) copolymers and methacrylate-acrylonitrile-butadiene-styrene (MABS) copolymers, optionally comprising polycarbonate and additives, lead to unwanted coating of the moulding tool in thermoplastic moulding by injection or extrusion, and accordingly claims compositions containing 0 mg/kg to 100 mg/kg of magnesium and/or calcium compounds. The emulsion polymers used in these compositions are precipitated by freeze precipitation on a flake ice machine rather than by the addition of magnesium sulfate solution, as is traditionally customary.
WO 98/28344 discloses a process for the continuous shear-induced coagulation of aqueous dispersions of grafted rubbers which overcomes the known disadvantage of precipitation by means of acids and/or salts as coagulants, namely that impurities often remain in the worked-up polymers and can detract from the properties of the product.
One problem of thermoplastic compositions comprising vinylaromatic copolymers prepared by the emulsion polymerization method, with salt inclusions resulting from the preparative process, is that mouldings produced therefrom are susceptible to the unwanted formation of surface defects (blistering) on exposure to moisture (e.g. condensed water or atmospheric moisture), especially at elevated temperatures, thereby restricting the use of such compositions in mouldings with a high gloss finish that demand a class A surface.